This invention relates generally to an apparatus and control system for working sheet material and more specifically to a machine, control system and method for laser welding employing a tilting mechanism and a seam tracking device.
It is common to employ a welding or cutting head in combination with an articulating robot arm or a moving gantry. However, most articulating robots suffer from a lack of precision and stability due to their inherent heavy moment arms extending a significant distance from their stationary bases. Furthermore, articulating robots usually only have a static movement accuracy of +/xe2x88x92100 microns, at best. This lack of accuracy and lack of stability detrimentally affects welding and cutting precision of a head mounted on the arm""s end. In contrast, gantries tend to be more stable and thus more accurate than articulating robots, but usually require significantly expensive concrete reinforcement within the gantry and difficult to achieve bridge machining tolerances. Notwithstanding, the gantry bridges are still somewhat imprecise due to machining tolerances.
To account for these tolerance and accuracy variations, optical seam tracking cameras and sensors have been recently used for various welding processes including gas metal arc welding, gas tungsten arc welding, plasma arc welding, submerged arc welding, flux-cord arc welding and laser beam welding. One such system is produced by Servo-Robot Inc. of Boucherville, Quebec, Canada. Other examples of such three dimensional vision seam sensing systems are disclosed in the following U.S. Pat. Nos: 5,168,141 entitled xe2x80x9cVision Guided Laser Weldingxe2x80x9d which issued to Tashjian et al. on Dec. 1, 1992; U.S. Pat. No. 4,969,108 entitled xe2x80x9cVision Seam Tracking Method and Apparatus for a Manipulatorxe2x80x9d which issued to Webb et al. on Nov. 6, 1990; and U.S. Pat. No. 4,621,185 entitled xe2x80x9cAdaptive Welding Apparatus having Fill Control Correction for Curvilinear Weld Groovesxe2x80x9d which issued to Brown on Nov. 4, 1986; all of which are incorporated by referenced herewithin. Most welding and cutting gantry devices employing optical sensing use the somewhat imprecise bridge as the Z axis (vertical) datum and accordingly automatically adjust a Z axis movement device. For articulating robots, each joint is moved to correlate Z axis changes in relation to the fixed base. A more traditional capacitive sensor has also been used to sense the distance between a welding head and the workpiece material. Such a capacitive sensor is disclosed within U.S. Pat. No. 5,428,280 entitled xe2x80x9cRobotic Movement of Object over a Workpiece Surfacexe2x80x9d which issued to Schmidt et al. on Jun. 27, 1995, and is incorporated by reference herewithin.
Laser welding and cutting with a CO2 laser or a Yag laser are also becoming commonplace. Laser welding is highly advantageous over other types of welding methods since laser welding allows for deep and high speed welding without requiring the difficult to handle and somewhat costly filler material. Furthermore, laser welding devices are significantly less expensive as compared to other types of welding equipment. However, traditional laser welding and cutting systems use indexed turning or steering mirrors to redirect the laser beam along a curved workpiece or seam. Such mirrored systems are disclosed within the following U.S. Pat. Nos.: 4,972,062; 4,855,564; 4,677,274; and 4,367,017. The necessity to rotate these types of mirrors about the laser beam requires complicated and costly computer programming while also being somewhat prone to damage in the workpiece environment. Furthermore, such redirecting mirrors also provide additional tolerance inaccuracies and tend to collect airborne debris. Additionally, a fixed pivot point positioned on or above the laser in traditional systems causes focal length and focal point inaccuracies in relation to the workpiece material when the laser is initially oriented or moved relative to the workpiece surface; this is the scenario disclosed in U.S. Pat. No. 5,190,204.
Moreover, a large pre-weld gap between adjacent sheet material edges typically prevents adequate laser butt welding. This problem is usually observed with pre-weld gaps having a material edge-to-edge dimension greater than 10 percent of the material thickness. The welding machine is shut off and the material is scrapped if the maximum gap width is present. Such a gap problem is recognized in U.S. Pat. No. 5,204,505. Accordingly, it would be desirable to provide a laser welding apparatus and control system employing an improved, accurate and automated mechanism for welding across a large seam gap.
In accordance with the present invention, the preferred embodiment of an apparatus and control system for working material employs a tilting mechanism which automatically tilts a head about a theoretical point disposed upon the sheet material. In another aspect of the present invention, the head is defined as a laser head and the pivot point corresponds with the laser beam focal point. A further aspect of the present invention provides a seam tracking device and control system which automatically adjust a welding head height relative to the sheet material as a datum rather than relative to a gantry or other structure supporting the welding head. In yet another aspect of the present invention, an optical seam tracking device is employed to automatically tilt the welding head along differing rotational planes. In still another aspect of the present invention, various axial slides are employed in combination with a gantry. A further aspect of the present invention uses a seam tracking device and microprocessor to track a welding seam and move a welding head predetermined amounts due to sensed pre-weld gap widths. Another aspect of the present invention uses an automated tilting mechanism for creating a tailored blank butt weld between dissimilar materials. An additional axis, controlled by the weld head processor may move or rotate a laser optic to orient a double spot optic to either increase welding speed or cover a larger area across the gap to ensure both metal sheets are melted. Also if two lasers are used (one for each spot), the weld head processor may selectively rise or lower each respective laser""s power, or change one or both lasers to pulse mode from continuous wave mode. Moreover, in a further aspect of the present invention, an automatically adjusting laser is used to create tailored blank welds. Generally, tailored blank welds are essentially butt welds between pre-cut or previously blanked-out sheets of material of similar or dissimilar materials types and/or thicknesses. A method of operating the present invention is also provided.
The laser welding apparatus and control system of the present invention have many advantages over traditional systems. For example, the automated nature of the present invention tilting mechanism, the closed-loop, actual condition sensing seam tracking device and the microprocessors allow for uninterrupted continuous welding along an arcuate seam regardless of even large seam gap widths. This automated tilting control system significantly lowers work material rejection, set-up time, operating time and operating costs while improving weld uniformity along material sheets having wide edge tolerances. Another advantage is the ability to rotate split laser beams in an automated manner. An integral electromagnetic laser optic member movement mechanism further aids welding across a large gap seam.
The present invention tilting mechanism is also very cost effective, evenly balanced, dimensionally stable and extremely precise. Furthermore, the present invention tilting mechanism is extremely robust and resistant to abuse and environmental debris. The apparatus of the present invention is further advantageous by accurately maintaining the focal length of the laser beam such that the focal point is always maintained upon the desired workpiece seam location regardless of the tilted positioning of the laser optic. Another advantage of the apparatus of the present invention is that it can be tilted to weld across a pre-weld gap between adjacent sheet material edges even if the pre-weld gap is between 10 and 15% of the sheet material thickness. The present invention apparatus offers another advantage by being especially suitable for use in tailored blank butt welding of dissimilar material thicknesses or material types by providing a mechanism for accurately tilting the laser head in relation to the stepped sheet material seam. By tilting the laser head of the present invention, the harmful reflected laser beam is angularly offset from the incident laser beam emanating from the welding or cutting laser head. As another advantage, the present invention uses the sheet material and seam as datums thereby allowing wider and less costly tolerances and stiffness requirements within the bridge and gantry. In general, gantry systems provide static precision movement within +/xe2x88x9225 microns thereby offering superior accuracy relative to articulating robots. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.